1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, and a printer, and more particularly, to a sheet guide member that guides a recording medium in an image forming apparatus.
2. Description of the Background Art
In general, in an image forming apparatus, a recording medium such as a sheet of paper on which an image is to be formed is guided through the image forming apparatus by one or more guide members. For example, a pair of guide plates is used to guide the recording medium to a transfer position at which a toner image, already formed on a surface of an image bearing member such as a photoreceptor or an intermediate transfer body, is transferred onto the recording medium.
During this transfer process, it is known that image defects tend to occur substantially at the end portion of the recording medium in the longitudinal direction. The image defect that occurs at the end portion of the recording medium because immediately after the trailing edge of the recording medium passes the upper guide plate, the trailing edge of the recording medium hits the belt-type intermediate transfer body, delivering an undesirable impact thereto that causes the recording medium to flap and thus causes toner constituting the toner image on the belt-type intermediate transfer member to scatter.
Various approaches have been proposed in an attempt to solve this problem. For example, one related-art image forming apparatus includes an approach guide plate to guide the recording medium to the transfer position of a belt-type intermediate transfer member. The toner image formed on the belt-type transfer member is then transferred onto the recording medium. The approach guide plate includes an upper guide plate and a lower guide plate. The upper guide plate is placed obliquely with respect to a transport direction of the belt-type intermediate transfer body. The upper guide plate includes a sheet member at the front end thereof so as to reduce the above-described impact of the intermediate transfer body against the recording medium.
With this configuration, image defects such as rear-end scattering or lateral-line scattering of the toner image can be prevented. As a result, it is possible to form a good toner image free of image defects on the recording medium. In particular, when an image is formed on a recording medium with some rigidity, such as a postcard, a good, defect-free toner image can be formed.
However, in such a related-art image forming apparatus, an amount of projection of the sheet member from the front end of the upper guide member is greatest at the center in the width direction, and gradually recedes toward the lateral edges in the width direction. Consequently, depending on the material of the sheet member, in particular when the recording medium is relatively rigid, it is difficult for the guide member including the sheet member to adequately suppress image defects at the end portion of the recording medium. Furthermore, image defects are still generated at the leading end of the recording medium.
Moreover, in terms of reducing the impact of flapping of the recording medium, material having relatively low bend-stiffness is suitable for the sheet member at the front end of the upper guide plate. It is known, however, that if the bend-stiffness of the sheet member is too low, a relatively rigid recording medium causes undesirable image degradation at the leading end of the recording medium. This is because the front end of such a relatively rigid recording medium advances to the belt-type intermediate transfer member while pushing and bending the sheet member. The recording medium, then, hits the belt-type intermediate transfer member. Immediately after that, flapping of the recording medium causes the toner constituting the toner image on the belt-type intermediate transfer member to scatter.
Conversely, in order to reduce undesirable image degradation at the leading end of the recording medium, it is desirable to use material having relatively high bend-stiffness for the sheet member.
Thus, as described above, the bend-stiffness of the material for the sheet member needs to be different for correcting image degradation at the leading end of the recording medium and image degradation at the trailing edge of the recording medium on the other.
The foregoing description pertains to problems relating to the guide member that guides the recording medium from the belt-type intermediate transfer body to a transfer position inside the image forming apparatus. Similar problems occur when the guide member guides the recording medium from a belt-type photoreceptor to the transfer position. Furthermore, similar problems occur when the guide member guides the recording medium from a drum-type image bearing member to the transfer position. Still further, immediately after the trailing edge of the recording medium passes through and becomes free of the guide member, when the position of the trailing edge of the recording medium changes substantially to the image bearing member side, image defects tend to occur. In addition, even if the trailing edge of the recording medium moves away from the image bearing member, problems still occur. This is because, for example, rapid positional change of the trailing edge of the recording medium causes flapping of the recording medium, generating vibration. Vibration adversely affects optical writing by an exposure unit in the image forming apparatus.
For reasons described below, not only the guide member of the image forming apparatus using the transfer method as described above but also the guide member of an image forming apparatus using a direct-recording method may not prevent undesirable vibration due to flapping of the recording medium.
Referring now to FIG. 11, there is provided a cross-sectional view of one example of a secondary transfer portion of a related-art image forming apparatus.
As illustrated in FIG. 11, the recording medium P is sandwiched between a transfer roller 131 and an opposing roller 132 in the secondary transfer portion, and between a pair of registration rollers 154. Because peripheral velocities of the pair of registration rollers 154 and the transfer roller 131 are different, the recording medium is deformed (bent) after the recording medium P is sandwiched between the transfer roller 131 and the opposing roller 132.
The apparatus further includes an upper guide member 161 located between the transfer roller 131 and the opposing roller 132, and the pair registration rollers 154. An end portion hj of the upper guide member 161 at a transfer nip side between the transfer roller 131 and the opposing roller 132 is positioned closer to the opposing roller 132 than to the transfer roller 131 with respect to a straight horizontal line L connected a registration nip between the registration rollers 154 to the transfer nip. A transfer guide roller 133 is provided in the inner loop of an intermediate transfer belt 120 and in the vicinity of the upstream of the opposing roller 132 in the direction of rotation of the belt.
With the configuration described above, there is no device to prevent the recording medium P from deforming toward the opposing roller 132. Furthermore, as the distance between the transfer nip portion where the transfer roller 131 and the opposing roller 132 meet and press each other and the end portion hj increases, the amount of the recording medium P between the nip portion and the end portion hj also increases and the rigidity of the recording medium P at this area decreases. As a result, the recording medium P deforms between the transfer nip portion and the end portion hj of the upper guide member 161 due to the difference in a peripheral velocity of the pair of the registration rollers 154 and the transfer roller 131 described above. When the recording medium P deforms, the recording medium P may contact the intermediate transfer belt 120. In this state, the recording medium P is immediately before being nipped by the transfer roller 131 or the opposing roller 132. The contact condition between the recording medium P and the intermediate transfer belt 120 is unstable, causing scattering of the toner constituting the toner image.